Assigning a value to a cell based on conditions of two other cells being equal

Okay I have tried searching and I am just not smart enough to figure out all this programming stuff so I am not exactly sure what to do, but I assume it is simple.

I have three columns, one is Purchase Size, one is Count Size, and one is Purchase Count.

I just need a formula that will look at the Purchase Size and Count Size and if they are equal automatically force the Purchase Count to be a value of 1. If they are not equal I need it to allow the user to input the correct amount.

Both the Purchase Size and Count Size columns utilize a list of possible selections.

It would be nice to also lock the ability to alter the 1 if the two columns are identical

Thank you very much